This invention relates generally to a method and apparatus for slush molding a thin shell, and, more particularly, for slush molding a thin shell having an outer layer and an inner layer.
The prior art includes a number of different methods for forming thin, resilient multi-layer trim component shells such as automotive instrument panel skins. It is often desirable, particularly in automotive interior trim applications, for the exterior xe2x80x9cclass Axe2x80x9d surfaces of such shells to have an appealing appearance and feel to automobile passenger compartment occupants. To provide an aesthetically pleasing outer class A surface, at least the outer layer of such multi-layered shells is often formed by slush molding methods. Slush molding involves casting a charge of polymer material against a heated mold surface that is shaped to form the desired shape and texture of the outer surface of a thin shell to be cast within the mold. The casting may be accomplished by attaching and sealing an open upper end of a charge box to an outer rim of an open end of the mold. The charge box is then inverted to allow the polymer material within the charge box to fall by gravity from the charge box and onto the heated mold surface. Once polymer material is applied to the heated mold surface, the charge box returns to its upright position and excess casting material returns to the charge box. The cast material is then allowed to melt on the heated surface, the mold surface is cooled, and the material is allowed to harden before removing it from the mold surface. The thin shell layers may be cast in a hot air module using a dry blend, usually in the form of powder or microspheres, of polymer such as polyvinyl chloride (PVC). The dry blend may include material for imparting a predetermined color to the shell.
Current slush molding methods include a xe2x80x9cdouble-castxe2x80x9d method that includes the casting of a first charge of polymer material on a heated mold surface as described above, then casting a second charge of polymer material on the inner surface of the layer formed by the first casting. Sufficient heat is transferred from the heated mold surface through the first layer to melt the second charge layer. The mold surface is then cooled to allow both layers to harden and bond to one another.
Double-cast slush molding processes are disclosed, for example, in U.S. Pat. No. 5,344,183, which issued Sep. 6, 1994 to Hersman et al.; U.S. Pat. No. 4,769,278, which issued Sep. 6, 1988 to Kamimura et al.; and U.S. Pat. No. 5,580,501, which issued Dec. 3, 1996 to Gallagher et al. Both Gallagher et al. and Kamimura et al. disclose the casting of a second charge comprising an expandable or foamable polymer powder. Hersman et al. disclose the casting of a second charge comprising reinforcing fibers impregnating a polymer material.
In addition, the assignee of the present invention has constructed and used a rotational apparatus for manufacturing thin walled hollow parts using a double-cast process. The apparatus is a modification of the apparatus disclosed in U.S. Pat. No. 4,722, 678 issued Feb. 2, 1988 to Wersosky and assigned to the assignee of the present invention. The apparatus includes a charge box supported on a movable cart that tracks with a guide mechanism. An electromechanical device shuttles the cart and charge box with the aid of the guide mechanism between a position directly below an open-ended mold for attachment to the mold and a position laterally displaced from the mold (i.e. perpendicular from the axis of rotation) for re-filling with polymer powder. The mold is mounted separately from the cart for rotation about trunnions on a support structure. The cart includes an elevator system that lifts the charge box into engagement with the open end of the mold when the cart is positioned below the mold. Once attached, the charge box is then inverted with the mold to a position over the mold to dispense its contents into the mold. The apparatus also includes a second cart and charge box that are sequentially shuttled into a position below the mold from a direction opposite the first cart and charge box and attached to the mold after the first charge box has been removed and displaced from the mold. From such equipment, two-layer slush molded shells have been manufactured from powdered polyvinyl chloride (PVC). The two-layer slush molded shells comprise a solid PVC outer layer and a foamed PVC inner layer made from virgin materials (i.e. has not been subjected to use other than required for its original manufacture). The materials further comprise the same PVC formulation with the same levels of ultraviolet stablization except that the PVC foam comprises a lower concentration of pigment resulting in a different color and a blowing agent to produce the foam, which makes the material unsuitable for use as the outer layer. The two layer slush molded shells further comprise an outer layer average thickness of 0.020 inches and an inner layer average thickness of 0.030 inches.
What is needed is an apparatus configured to execute a double cast slush molding method without the expense of multiple charge boxes and associated expense of added carts, rails and elevator systems. What is also needed is a method for slush molding a double-layer shell or skin using at least a proportion of less expensive materials without compromising the high-quality appearance of an outer class-A surface of the shell or skin.
In accordance with this invention, an apparatus for molding a thin shell having an outer layer and an inner layer is provided that includes at least two tubs configured to hold charge materials for delivery to a mold cavity. The tubs are simultaneously rotatable around a horizontal axis to an orientation where the charge materials will dispense from the tubs. The tubs have openings configured to be alternately blockable to allow the tubs to alternately dispense charge material into the mold cavity.
According to another aspect of the invention, the apparatus includes a support structure supported for rotation around a horizontal axis with at least two tubs supported on the support structure and configured to hold charge materials for delivery to a mold cavity. The tubs are rotatable with the support structure around the horizontal axis to an orientation where the charge materials will dispense from the tubs. The tubs are also alternately movable on the support structure to a dispensing position. The support structure is configured to block the opening of a tub not in the dispensing position and to unblock the opening of a tub in the dispensing position. This allows the tubs to alternately dispense their charge material into the mold cavity.
According to another aspect of the invention, the first one of the two tubs is movable between a first outer position and the dispensing position while the second one of the two tubs is movable between a second outer position and the dispensing position. The support structure is configured to block the tub opening of the first tub in the first outer position and block the tub opening of the second tub in the second outer position. The support structure is also configured to allow charge materials to flow from the tub in the dispensing position.
According to another aspect of the invention, a diverter valve connectable between the tubs and the mold cavity. The diverter valve is configured to alternately open a passage from each of the tubs to the mold cavity to allow charge materials from the respective tubs to be selectively dispensed into the mold cavity.
According to another aspect of the invention, the diverter valve includes a separator blade pivotally mounted within a chute that extends between the mold cavity and the two tubs. The separator blade is pivotally movable between a first position and a second position within the chute. In the first position the separator blade is positioned to admit the flow of material from the first tub through the chute and to block the flow of material from the second tub through the chute. In the second position the separator blade is positioned to admit the flow of material from the second tub through the chute and to block the flow of material from the first tub through the chute.
According to another aspect of the invention, a method for molding a thin shell having an outer layer and an inner layer is provided. The method includes providing a mold having a mold surface configured to complement the desired shape of the shell to be molded and heating the mold surface. A first polymer material is provided in a first tub having a first tub opening and a second polymer material is provided in a second tub having a second tub opening. The second tub opening is blocked and the tubs and the mold are tipped until at least a portion of the. first polymer material dispenses from the first tub onto the mold surface to form an outer layer. The tubs and the molds are then righted and the second tub opening is opened while the first tub opening is blocked. The tubs and the mold are then tipped until at least a portion of the second polymer material dispenses from the second tub onto at least a portion of the outer layer to form an inner layer. The mold surface is then cooled, the inner and outer layers are bond together and the shell is removed from the mold.
According to another aspect of the invention, the method for molding a thin shell having an outer layer and an inner layer including the step of providing a first polymer material in a first tub includes the steps of moving the first tub to a filling position on a support structure and filling the first tub with the first polymer material and the step of providing a second polymer material in a second tub includes the steps of moving the second tub to a filling position and filling the second tub with the second polymer material.
According to another aspect of the invention, the method for molding a thin shell having an outer layer and an inner layer including the steps of providing a first polymer material in a first tub and providing second polymer material in a second tub includes the steps of connecting the tubs to a diverter valve to provide a polymer material flow path from one of the tubs while blocking polymer material from flowing from the other of the tubs, operating the diverter valve to provide a flow path for the first polymer material from the first tub while blocking the flow of the second polymer material from the second tub and operating the diverter valve to provide a flow path for the second polymer material out of the second tub while blocking the flow of the first polymer material from the first tub.
According to another aspect of the invention, the method for molding a thin shell having an outer layer and an inner layer including the step of connecting the tubs to a diverter valve is preceded by an additional step of providing a diverter valve comprising a separator blade pivotally mounted within a chute. The chute is configured to releasably connect a mold cavity to the two tubs and to provide a polymer material flow path from the tubs to the mold cavity. The separator blade is pivotally movable between a first position and a second position within the chute. The separator blade in the first position admits the flow of material from the first tub through the chute and blocks the flow of material from the second tub through the chute. The separator blade in the second position admits the flow of material from the second tub through the chute and blocks the flow of material from the first tub through the chute.
According to another aspect of the invention, the method for molding a thin shell having an outer layer and an inner layer including the steps of operating the diverter valve to provide a flow path for the first polymer material includes the step of pivoting the separator blade to the first position, and the step of providing a layer of the second polymer material on the layer of the first polymer material by operating the diverter valve includes the step of pivoting the separator blade to the second position.
According to another aspect of the invention, a thin shell for an automotive trim panel having an outer layer and an inner layer is disclosed, where the outer layer comprises a first polymer material and an inner layer comprising a second polymer material. The inner layer at least partially covers the inner surface of the outer layer and is concealed from view by vehicle occupants.
According to another aspect of the invention, a thin shell for an automotive trim panel is disclosed, said shell having an outer layer and an inner layer, the shell comprising an outer layer comprising a first polymer material and an inner layer comprising a second polymer material, the second polymer material further comprising a polymer material at least a portion of which comprises a formed article prior to its use as the second polymer material, and the inner layer at least partially covering the inner surface of the outer layer and concealed from view by vehicle occupants.
According to another aspect of the invention, a thin shell for an automotive trim panel is disclosed, said shell having an outer layer and an inner layer, the shell comprising an outer layer comprising a first polymer material, an inner layer comprising a second polymer material, the second polymer material further comprising a mixture of two or more different polymer formulations, and the inner layer at least partially covering the inner surface of the outer layer and concealed from view by vehicle occupants.
According to another aspect of the invention, a thin shell for an automotive trim panel is disclosed, said shell having an outer layer and an inner layer, the shell comprising an outer layer comprising a first polymer material, an inner layer comprising a second polymer material, the second polymer material further comprising a polymer formulation which is more susceptible to ultraviolet degradation than the first polymer material, and the inner layer at least partially covering the inner surface of the outer layer and concealed from view by vehicle occupants.
According to another aspect of the invention, a thin shell for an automotive trim panel is disclosed having an outer layer and an inner layer wherein the outer layer comprises an average thickness in a range between and including 0.005 inches to 0.025 inches.